This invention relates to an improved oil shale retorting process and a separation system using iron oxide impregnated porous heat carriers.
U.S. Pat. No. 3,844,929 provides an oil shale retorting process using porous pellets having a surface area of at least 10 square meters per gram and a size ranging from approximately 0.04 centimeter (0.055 inch) to approximately 1.27 centimeters (0.5 inch). In this process, a combustible deposition is formed on the pellets. This deposition is burned to heat the pellets to a temperature between 538.degree. C. (1000.degree. F.) and 816.degree. C. (1500.degree. F.). The heated pellets are mixed with crushed oil shale. The heat from the pellets helps to convert the kerogen in the oil shale to oil and gas and produces a mixture of pellets and spent shale solids. The pellets are separated from the spent shale solids so that the porous pellets may be heated and recycled through the retorting process. The pellets must be separated in a dry system wherein dust and other emissions into the atmosphere are controlled. In addition, the mixture of pellets and spent shale are usually separated at relatively high temperatures, e.g., 204.degree. C. ( 400.degree. F.) to 538.degree. C. (1000.degree. F.). A commercial oil shale plant using porous pellets may retort as much as 50,000 metric tons (about 55,000 short tons) to 118,000 metric tons (about 130,000 short tons) of raw crushed oil shale per day with varying oil yields per ton. This will require recovery and recycle of one to three times as many tons of the heat-carrying pellets. Several methods of separating the solids have been proposed. In U.S. Pat. No. 3,803,021, a combination elutriation-size screening separating system is used to recover the heat-carrying solids. In co-pending Application Ser. No. 749,505, filed Dec. 10, 1976, now U.S. Pat. No. 4,118,309 entitled "Separation and Recovery of Heat Carriers in an Oil Shale Retorting Process", and owned by a common assignee, a continuously restored inclined surface is used for assisting in separation of heat-carrying solids from the spent shale solids. In co-pending Application Ser. No. 858,578, filed Dec. 12, 1977, entitled "Magnetic Separation of Heat Carrying Solids from Spent Oil Shale", and owned by a common assignee, a ferromagnetic material is added to the heat carriers and the solids passed through a magnetic separator. At the temperatures involved in retorting oil shale, the addition of iron into the heat carriers is likely to adversely affect the strength properties of the heat carriers. Moreover, the heat carriers may be handled in a way where residual magnetism of the iron causes handling problems.
The oil produced by retorting oil shale contains metal impurities like arsenic which poison hydrotreating catalysts and are removed prior to substantial hydrogenation of the oil.